Characterization of plastic instability steps occurring in depth-sensing indentation tests

Plastic instabilities in solid solution Al–Mg and supersaturated Al–Zn–Mg–(Cu) alloys were investigated by depth-sensing microhardness tests. Experimental results show that in the case of Al–Mg alloys the characteristics of the plastic instability depend only on the solute concentration. In the case of age-hardenable Al–Zn–Mg–(Cu) alloys, however, although plastic instabilities were observed during indentation following the quenching, the development of the instability steps is strongly influenced by Guinier–Preston zone formation in the early stage of natural aging. After a certain time, ti, of aging the steps disappear completely. The value of ti obtained for the Cu-containing alloy was found to be considerably longer than that obtained for the ternary Al–Zn–Mg alloy. This fact – together with the smaller initial rate of hardening in the Cu-containing alloy – indicates that the addition of Cu retards the early part of the decomposition process at room temperature in this alloy system.